VISHAY SEMICONDUCTORS
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Optocouplers and Solid-State Relays
Application Note 33
VO3526 Power Phototriac Design Considerations
The VO3526 power phototriac (also termed a solid-state
relay in the industrial and process control industries) is
designed to be used as a solid-state AC switch for loads of
up to 1 A. As with all power devices, heat dissipation should
be kept in mind when using the VO3526 to its maximum load
capacity. To achieve maximum heat dissipation from the
device through the leads a good PCB layout is necessary,
since using a heat sink may conflict with safety agency
requirements.
In this application note we examine the impact of the
following factors that will influence designs using the
VO3526 to drive a resistive or inductive load:
HEAT DISSIPATION
There are three heat transfer modes for VO3526
1. Conduction through the leads to the PCB: because of its
lower thermal resistance (RBA) most of the heat (~ 70 %
to 75 %) is conducted through the device leads to the PCB
2. Radiation through the device case
3. Convection through the device case
The convection and radiation phenomena are associated
with RCA (case-to-ambient thermal resistance), which is
typically an order of magnitude larger than RBA. Thus these
modes do not transfer as much heat out of the package.
• Load current derating over ambient temperature
Radiation and convection
through case
• Heat dissipation
• PCB design considerations
25 %
• Snubber circuit
• LED degradation
LOAD CURRENT DERATING OVER AMBIENT
TEMPERATURE
75 %
Conduction
through board
1.2
4 Layer
Fig. 2 - Heat Transfer Mode in %
Load Current (A)
1.0
For a detailed explanation of the thermal model, please refer
to Vishay’s Thermal Characteristics of Power Phototriac
Application Note.
0.8
0.6
2 Layer
PCB DESIGN CONSIDERATIONS
0.4
Pin 4 to 7
0.2
21510
0
20
40
60
80
31 %
100 120
Pin 2, 3
Ambient Temperature (°C)
16 %
Fig. 1
12 %
Pin 9
To avoid exceeding the maximum die junction temperature,
the load current must be reduced at ambient temperatures
higher than 25 °C.
12 %
Pin 15
11 %
18 %
Pin 11
Pin 13
21513
Fig. 3 - Heat Through Leads in %
Rev. 1.1, 19-Oct-11
Document Number: 81937
1
For technical questions, contact: optocoupleranswers@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
0
- 40 - 20
Application Note 33
www.vishay.com
Vishay Semiconductors
VO3526 Power Phototriac Design Considerations
The maximum load current value depends whether a 2 layer
or 4 layer PCB is used. Since a 4-layer PCB will do a better
job of dissipating heat, a higher load current of up to 1 A can
be achieved (see figure 1)
The PCB design has a major impact on board-to-ambient
thermal resistance. Use the proper pad size, layout, and
thickness of copper traces to maximize heat dissipation
through the PCB. The size of the pads and copper area on
both top and bottom sides should be as big as possible
while keeping a minimum safe distance between input and
output pins. Additionally thermal vias could be added to the
layout to improve upon heat dissipation.
The following is an example of a PCB pads design.
5V
R1
150
VO3526
13
2
3
U1
RS
47
11
Inductive
CS
load
0.022 µF
Snubber
circuit
Hot
220/240
VAC
Neutral
21515
Fig. 5
The value of CS and RS depends on the application and
should be adjusted accordingly. Locate the snubber circuit
as close to the VO3526 as possible.
LED DEGRADATION
All LED emitters degrade over time. This degradation is
accelerated by temperature and LED current. Therefore in
order to maintain proper operation, it is good practice to
provide extra headroom for the minimum triggering current
required.
SUMMARY
21514
Fig. 4
Using good design practices to lay out the PCB for
maximization of heat dissipation through the device pins to
the PCB, the VO3526 can be used to drive resistive and
inductive loads of up to 1 A. In the case of inductive loads,
a snubber may be needed to avoid unwanted turn-on of the
VO3526 in the off-state.
As conduction is the major mode of heat transfer, all pins,
including pins 4 through 7, should be soldered to the PCB.
To avoid safety certification violations, please note that the
input and output sides have to be kept isolated. Sockets
should not be used as they will reduce the effective rate of
thermal conduction of the package.
APPLICATION NOTE
SNUBBER CIRCUIT
Static dV/dt is the minimum value of the rate-of-rise of the
voltage which causes switching on of the VO3526 when it is
in off state. When driving inductive loads such as motors
and solenoids, the device should be off when its load
current is less than the holding current (IH) and the LED
forward current (IF) drops below phototriac's trigger current
threshold (IFT). However, if the slew rate is faster than the
VO3526 rated dV/dt, the device may turn on again. A
snubber circuit is used to overcome this problem.
Rev. 1.1, 19-Oct-11
Document Number: 81937
2
For technical questions, contact: optocoupleranswers@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000